Cross regulation circuits are widely applied to the field of industrial automation, for example man-machine interaction interfaces on large-scale industry instruments. Transformers are usually used for creating multiple output circuits of security isolation. Especially in the low-voltage industrial field, multiple output isolation circuits based on flyback topology are used commonly. This is because they have low power consumption and it is easy to obtain multiple power outputs.
However, the problem that needs to be taken into account is how to carry out cross regulation, since an output voltage value of one output branch is greatly influenced by other branches, for example factors such as load change of other branches. In particular, in the application scenario of complete isolation, cross regulation is particularly important. For a hardware developer, it generally takes a long time to carry out cross regulation.
FIG. 1 is a circuit connection diagram of an isolation circuit for multiple outputs in the prior art. As shown in FIG. 1, the left part of the circuit is provided with a transformer circuit. An input voltage Vdc and output voltages V1 and V2 should be based on the transformer principle and determined according to the number of turns of the three on an iron core. However, since two output branches where the output voltages V1 and V2 are located are closely arranged, when the load of the branch where the output voltage V1 is located changes, and in order to carry out cross regulation so as to obtain an accurate output voltage V2, a low dropout regulator 101 is arranged on the upstream of the circuit of the output voltage V2 for stabilizing the output voltage V2 in the prior art.
Instead, there are some drawbacks, such as high costs, in the prior art where a low dropout regulator is used for regulating an output voltage. In addition, the power consumption of a circuit is high, particularly when there are a lot of loads on the branches of the output voltages V1 and V2.
As shown in FIG. 2, in the prior art, a dummy load device 102 that is connected in parallel and grounded is further arranged on the upstream of the output voltage V2 to consume the redundant voltage of the branch of the output voltage V2. Instead, the dummy load is not suitable to all situations, it is especially and only suitable to the case where a load of the branch of the output voltage V1 is great and a load of the branch of the output voltage V2 is small. In addition, the dummy load 102 may increase system energy loss.
The greater the load of the branch of the output voltage V1 is, the larger the area of a printed circuit board (PCB) occupied by the dummy load 102 is, and this is taken into account based on the factor of heat dissipation.
As shown in FIG. 3, in the prior art, a “sandwich winding method” in which an input and an output of the transformer circuit are differently wound on an iron core is further used for carrying out cross regulation, where Np represents the number of turns of primary coils of the transformer input, Ns1 represents the number of turns of secondary coils corresponding to the output voltage V1 of the transformer, and Ns2 represents the number of turns of secondary coils corresponding to the output voltage V2 of the transformer.
As shown in FIG. 3, on a framework 103, one half of the number of turns Np/2 of the primary coils are firstly wound, then secondary coils Ns1 and Ns2 are wound in sequence, and finally one half of the number of turns Np/2 of the primary coils are wound. Drawbacks of the “sandwich winding method” exist in that it has to balance the cross regulation and electromagnetic compatibility, and the costs of the transformer may also be increased. In addition, the framework 103 is generally very long, and in the case where the number of turns of the secondary coils is few, it is hard to wind same uniformly.